The appearance of eight strains of alfalfa mosaic virus in alfalfa leaves and their effect on the ultrastructure of infected cells

Abstract

Eight strains of alfalfa mosaic virus (AMV) were studied by electron microscopy in alfalfa (Medicago sativa L.) leaf tissue and after purification. The virus occurred in the cytoplasm and occasionally in the vacuoles of mesophyll and vascular parenchyma cells; it was not associated with cell organelles. One strain of AMV (U5) did not incite symptoms in the alfalfa leaves and caused no ultrastructural damage to the infected cells. Two strains (U10 and U21) caused no symptoms in alfalfa, but the tonoplast of infected cells was not closely attached to the cytoplasm and floated in the vacuole; cell organelles were not damaged. The other five strains of AMV (F1, NY1, R6, B1, and W1) regularly or occasionally produced symptoms in alfalfa leaves. In leaves that were symptomless, as well as in the apparently healthy parts of leaves with symptoms of AMV infection, there was no apparent ultrastructural damage to the infected cells. Within the part of a leaf where there were symptoms, the tonoplast was detached from the cytoplasma and was folded within the vacuole in various patterns, along with bits of cytoplasm and virus particles. Cell organelles were often found in various stages of disintegration. Three different aggregations of the virus were recognized. The eight AMV strains were grouped into three general classes on the basis of the range in virus particle sizes. Mycoplasm was not associated with any of the AMV strains.